The human processing of visual signals has been studied in the framework of the Theory of Signal Detectability. Previous failures of the theory to adequately describe visual performance spring from 3 areas. 1) Failure of instrumentation to deliver requisite stimuli. 2) Failure to take advantage of the full scope of the theory. 3) Failure of the theory to address any but the simplest problems. Using instrumentation already developed to synthesize appropriate stimuli, I intend to extend my previous theoretical and experimental investigations into the following areas: 1) continued study of the limit posed by quantum fluctuations on visual performance, expecially at photopic levels where the possibility has not been accorded much attention before, 2) re-investigate the estimates of dark light and quantum efficiency in foveal cones based upon the results in (1); 3) investigate psychometric functions for elementary stimuli other than increments (e.g., for decrements, for temporally or spatially separated increment-decrement pairs; 4) continued expansion of the theory of signal detectability to embrace systems with independent summing and differencing channels; 5) characterization of normal subjects' foveal summing and differencing channel complements in all meridians, and 6) investigate the characteristics of the deficit in "Meridional Amblyopia" due to early uncorrected astigmatism. These six items focus on the issue of limitations on visual performance in health and disease. Such questions as: Is quantum catching ability impaired? Are summing and/or differencing channels impaired? Is extra visual noise or malfunctioning gain control responsible for a deficit? can now be answered. These are important issues in many areas of visual anomaly. The tools arrayed here are unique in their promise to deliver answers, and the method of getting answers should thus have more general applicability. The methods to be used are standard classical and modern psychophysical procedures. Two alternative forced choice experiments and conventional method of limits comprise the bulk of procedures to be used.